Machines such as, for example, wheel loaders, track type tractors, and other types of heavy machinery can be used for a variety of tasks. These machines include a power source, which may be, for example, an engine, such as a diesel engine, gasoline engine, or natural gas engine that provides the power required to complete such tasks. To effectively maneuver the machine during performance of such tasks, the machines also include a transmission that is capable of transmitting the power generated by the engine to various drivetrain components of the machine over a wide range of conditions.
For example, such machines commonly use a continuously variable transmission (“CVT”) to direct engine torque to traction devices, such as wheels or tracks, that propel the machine. A CVT is capable of providing a desired output torque to such components, at any speed within its operating range, by continuously changing the ratio of the transmission. The engine and/or the CVT may also be used to assist in braking the machine. For example, during operations in which the machine is required to change travel directions at relatively high load, the engine and the CVT may be configured to provide a retarding torque to the traction devices in order to stop the machine.
For example, upon loading an exemplary wheel loader bucket with material from a pile, the wheel loader may be directed to travel in a reverse direction away from the pile. While traveling in the reverse direction under such a relatively high load, the wheel loader may be controlled to stop, and to move in a forward direction so that the material can be carried to a dump truck, or other unload location. Although engine speed may be increased during such high-load direction changes to provide retarding torque to the traction devices via the CVT, the combined retarding torque provided by the engine and CVT may be inadequate to absorb all of the energy associated with braking the machine in a timely manner. As a result, the efficiency of the machine during repeated loading cycles may suffer. Although additional loads and/or torque demands may be placed on the CVT and/or the engine to further assist with machine braking, known control systems are not configured to manage the combination of such loads such that the load cycle efficiency of the machine is maximized. Instead, traditional power systems including an engine and a CVT are controlled by measuring engine speed, and changing the ratio of the transmission to keep the engine within a defined speed range. Such systems typically focus on protecting machine components from damage caused by engine overspeed.
For example, U.S. Pat. No. 6,385,970 to Kuras et al. discloses a system that includes an engine, a hydraulic CVT, and a control system in communication with the engine and the CVT. The control system of the '970 patent is paired with a hydro-mechanical drive system that is operable to sense engine speed and create an output speed signal. The control system is further operable to compare the engine speed signal to an underspeed value and produce an error signal. The error signal is used to produce a command signal that controls the transmission ratio to manage the load on the engine.
While the control system of the '970 patent may incorporate various strategies to increase the amount of engine and/or CVT retarding torque available for braking the machine, and to protect the engine and CVT from overspeed damage during machine braking, the control system does not seek to minimize the time required to brake the machine during various loading and unloading cycles. As a result, the control system of the '970 patent does not optimize a loading cycle efficiency of the machine.
The present disclosure is directed towards overcoming one or more of the problems as set forth above.